In recent years, rising fuel prices, concerns about greenhouse gas emissions and political instability in oil-producing regions have generated an increase in the interest of producing fuels from renewable biomass resources (“biofuels”). At the forefront of this effort has been a surge in the production of bioethanol. In a typical bioethanol production strategy, biomass is processed to release sugars that are then fed to microorganisms that ferment the sugars to ethanol. While both bacteria and yeasts are capable of ethanol production, the workhorse of industrial ethanol production to date has been yeast from the genus Saccharomyces, especially Saccharomyces cerevisiae (S. cerevisiae).
Processes have been described in which ethanol has been produced from simple sugars released from sugar cane and sugar beets, from starches isolated from corn and from sugars liberated from plant-derived cellulose (termed “cellulosic ethanol”). The latter typically involves the use of S. cerevisiae strains that have been genetically engineered to ferment pentose (5-carbon) sugars such as xylose (U.S. Pat. Nos. 4,368,268, 4,511,656, 4,663,284, 5,789,210, 5,866,382, 6,071,729, 6,582,944, 7,226,735, and 7,285,403) and arabinose (European Patent No. EP 1499708) in addition to glucose. While these processes are all viable, each also has drawbacks. Sugar cane, sugar beets and corn are natural resources that are also in the human food chain. Utilization of these resources for bioethanol generation has negatively impacted food prices. Additionally, each of the current methods employs large-scale exploitation of agricultural resources to generate the biomass needed for ethanol production. Thus, ethanol manufacturing is currently limited to geographic regions that are optimal for growth of energy crops. In one 2005 estimation, devoting all current U.S. corn production to generation of bioethanol would only offset U.S. petroleum use by 12% (Hill et al., Proc. Nat. Acad. Sci. USA 103:11206-11210 (2006)). Thus, to augment the current strategies for ethanol production, other types of sugar-rich biomass need to be considered for conversion to biofuels such as ethanol.
Chitin is generally regarded as the second most abundant polysaccharide in nature after cellulose. It is an unbranched β1,4-linked polymer of N-acetylglucosamine (GlcNAc) that is structurally quite similar to cellulose. It is a major component of insect exoskeletons, the shells of invertebrate crustaceans and cell walls of yeast and filamentous fungi. It has been estimated that >1011 tons of chitin are produced annually in the biosphere. In marine waters, the annual chitin production from a single genus of zooplankton (copepods) is estimated to exceed billions of tons (Keyhani & Roseman, Biochimica et Biophysica Acta 1473: 108-122 (1999)). Additionally, hundreds of thousands of metric tons of chitin-containing shellfish waste are generated as a byproduct of the shrimp and crab industries annually, much of which is dumped back into the ocean for disposal or used for fertilizer. Finally, chitin is a significant component of fungal cell walls along with polymers of other fermentable sugars such as mannose (mannan) and glucose (glucan). Large amounts of fungal biomass are generated in various industrial processes for production of commercially important metabolites, industrial enzymes, and protein therapeutics, as well as brewing processes and even ethanol production. Additionally, fungi can be propagated on industrial waste products such as cheese whey (Maullu et al., Appl. Environ. Microbiol. 65(6):2745-7 (1999); Fonseca et al., Appl. Microbiol. Biotechnol. 79(3):339-54 (2008), waste office paper (Ikeda et al., Bioresource Technol. 97(8):1030-5 (2006) and shellfish waste (Rattanakit et al., J. Bioscience and Bioeng. 93(6):550-6 (2002); Rattanakit et al., J. Bioscience and Bioeng. 95(4):391-396 (2003)). Thus, chitin is an abundant renewable source of sugars that can be considered in biofuel production strategies. However, despite its abundance, a significant drawback to its metabolic conversion to ethanol is the inability of various ethanol producing yeasts, especially yeast of the genus Saccharomyces, to metabolize and ferment the monomeric saccharide of chitin, the amino-sugar GlcNAc.